Exhaust gas treatment catalyst

ABSTRACT

Described is a catalyst composition suitable for use as a selective catalytic reduction catalyst, including small-pore molecular sieve particles having a pore structure and a maximum ring size of eight tetrahedral atoms and impregnated with a promoter metal, and metal oxide particles dispersed within the small-pore molecular sieve particles and external to the pore structure of the small-pore molecular sieve particles, wherein the metal oxide particles include one or more oxides of a transition metal or lanthanide of Group 3 or Group 4 of the Periodic Table. A method for preparing the catalyst, a method for selectively reducing nitrogen oxides, and an exhaust gas treatment system are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International App. No.PCT/IB2016/056906; filed Nov. 16, 2016, which International Applicationwas published by the International Bureau in English on May 26, 2017,and claims priority to U.S. Provisional Application No. 62/256,258,filed Nov. 17, 2015, each of which is incorporated by reference in theirentirety and for all purposes.

TECHNICAL FIELD

The present invention is directed to an exhaust gas purifying catalyst,method of making the catalyst, and methods for its use. Moreparticularly, the invention relates to selective catalytic reductioncatalysts including a small-pore molecular sieve containing a promotermetal.

BACKGROUND

Over time, the harmful components of nitrogen oxides (NO_(x)) have ledto atmospheric pollution. NO_(x) is contained in exhaust gases such asfrom internal combustion engines (e.g., automobiles and trucks), fromcombustion installations (e.g., power stations heated by natural gas,oil, or coal), and from nitric acid production plants.

Various methods have been used in the treatment of NO_(x)-containing gasmixtures. One type of treatment involves catalytic reduction of nitrogenoxides. There are two processes: (1) a nonselective reduction processwherein carbon monoxide, hydrogen, or a lower hydrocarbon is used as areducing agent, and (2) a selective reduction process wherein ammonia orammonia precursor is used as a reducing agent. In the selectivereduction process, a high degree of removal with nitrogen oxide can beobtained with a small amount of reducing agent.

The selective reduction process is referred to as a SCR process(Selective Catalytic Reduction). The SCR process uses catalyticreduction of nitrogen oxides with ammonia in the presence of atmosphericoxygen with the formation predominantly of nitrogen and steam:

4NO+4NH₃+O₂→4N₂+6H₂O  (standard SCR reaction)

2NO₂+4NH₃→3N₂+6H₂O  (slow SCR reaction)

NO+NO₂+NH₃→2N₂+3H₂O  (fast SCR reaction)

Catalysts employed in the SCR process ideally should be able to retaingood catalytic activity over the wide range of temperature conditions ofuse, for example, 200° C. to 600° C. or higher, under hydrothermalconditions. Hydrothermal conditions are often encountered in practice,such as during the regeneration of a soot filter, a component of theexhaust gas treatment system used for the removal of particles.

Molecular sieves such as zeolites have been used in the selectivecatalytic reduction (SCR) of nitrogen oxides with a reductant such asammonia, urea, or a hydrocarbon in the presence of oxygen. Zeolites arecrystalline materials having rather uniform pore sizes which, dependingupon the type of zeolite and the type and amount of cations included inthe zeolite lattice, range from about 3 to 10 Angstroms in diameter.Zeolites having 8-ring pore openings and double-six ring secondarybuilding units, particularly those having cage-like structures, haverecently found interest in use as SCR catalysts. A specific type ofzeolite having these properties is chabazite (CHA), which is a smallpore zeolite with 8 member-ring pore openings (˜3.8 Angstroms)accessible through its 3-dimensional porosity. A cage like structureresults from the connection of double six-ring building units by 4rings.

Metal-promoted zeolite catalysts including, among others, iron-promotedand copper-promoted zeolite catalysts, for the selective catalyticreduction of nitrogen oxides with ammonia are known. Iron-promotedzeolite beta has been an effective commercial catalyst for the selectivereduction of nitrogen oxides with ammonia. Unfortunately, it has beenfound that under harsh hydrothermal conditions, for example exhibitedduring the regeneration of a soot filter with temperatures locallyexceeding 700° C., the activity of many metal-promoted zeolites beginsto decline. This decline is often attributed to dealumination of thezeolite and the consequent loss of metal-containing active centerswithin the zeolite.

Metal-promoted, particularly copper promoted aluminosilicate zeoliteshaving the CHA structure type have recently solicited a high degree ofinterest as catalysts for the SCR of oxides of nitrogen in lean burningengines using nitrogenous reductants. This is because of the widetemperature window coupled with the excellent hydrothermal durability ofthese materials, as described in U.S. Pat. No. 7,601,662. Prior to thediscovery of metal promoted zeolites described in U.S. Pat. No.7,601,662, while the literature had indicated that a large number ofmetal-promoted zeolites had been proposed in the patent and scientificliterature for use as SCR catalysts, each of the proposed materialssuffered from one or both of the following defects: (1) poor conversionof oxides of nitrogen at low temperatures, for example 350° C. andlower; and (2) poor hydrothermal stability marked by a significantdecline in catalytic activity in the conversion of oxides of nitrogen bySCR. Thus, the invention described in U.S. Pat. No. 7,601,662 addresseda compelling, unsolved need to provide a material that would provideconversion of oxides of nitrogen at low temperatures and retention ofSCR catalytic activity after hydrothermal aging at temperatures inexcess of 650° C.

Even though the current catalysts exhibit excellent properties, there isa continuing desire to reduce N₂O make during the SCR reaction.Accordingly, a catalyst is needed with improved NO_(x) conversionefficiency and lower N₂O make relative to the current technologies.

SUMMARY

The invention relates to a catalyst composition suitable for use as aselective catalytic reduction catalyst, comprising an intimate mixtureof small-pore molecular sieve particles having a pore structure and amaximum ring size of eight tetrahedral atoms and impregnated with apromoter metal, and metal oxide particles comprising one or more oxidesof a transition metal or lanthanide of Group 3 or Group 4 of thePeriodic Table. It has been discovered that certain embodiments of acatalyst composition comprising molecular sieve particles with metaloxide particles dispersed therein (but external to the pore structure ofthe small-pore molecular sieve particles) can provide enhanced NO_(x)reduction at low and/or high temperature, as well as reduced N₂O make atlow and/or high temperature, as compared to a conventionalmetal-promoted molecular sieve containing no metal oxide particles orcontaining only metal oxide derived from minimal amounts of certainbinder materials. The metal oxide is typically present in an amount inthe range of about 1 to about 15% by weight, on an oxide basis, based onthe total weight of the washcoat.

The metal oxide particles typically comprise a metal oxide selected fromthe group consisting of zirconia, alumina, ceria, hafnia, yttria, andcombinations thereof. In certain embodiments, the metal oxide particleshave an average particle size in the range of about 10 nm to about 500nm and/or a D₁₀ particle size greater than ten times larger than a poreopening of the molecular sieve. In one embodiment, the metal oxideparticles have a D₁₀ particle size of about 10 nm or greater.

The catalyst composition can include a small-pore molecular sieve havinga d6r unit. Exemplary small-pore molecular sieves have a structure typeselected from AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, LTN, MSO, SAS,SAT, SAV, SFW, and TSC. In one embodiment, the molecular size is SSZ-13.

The catalyst composition will typically include a promoter metalselected from the group consisting of Cu, Co, Ni, La, Mn, Fe, V, Ag, Ce,Nd, Pr, Ti, Cr, Zn, Zn, Nb, Mo, Hf, Y, W, and combinations thereof. Inexemplary embodiments, the promoter metal comprises Cu or Fe orcombinations thereof. A typical amount of promoter metal is about 1 toabout 10% by weight, based on the total weight of the molecular sieve,such as about 2 to about 5% by weight. In another aspect, the inventionprovides a catalyst article comprising a substrate selected from aflow-through monolith, a wall-flow filter, a foam, or a mesh, wherein acatalyst composition according to any embodiment of the presentdisclosure is adhered as a washcoat layer on the substrate. In certainembodiments, the catalyst article of the invention is characterized byan N₂O make that is at least 10% by weight lower (or at least 15% loweror at least 20% lower or more) as compared to a catalyst articlecomprising a washcoat with the same catalyst composition at the sameloading but without metal oxide particles dispersed within thesmall-pore molecular sieve particles.

In yet another aspect, the invention provides a method for selectivelyreducing nitrogen oxides (NO_(x)), the method comprising contacting anexhaust gas stream containing NO_(x) with a catalyst composition orcatalyst article according to any embodiment of the present disclosure.In certain embodiments, the amount of N₂O produced as a byproduct isreduced in the method of the invention as compared to processespracticed with certain conventional catalyst compositions and catalystarticles. For example, in one embodiment, the amount of N₂O produced asa byproduct in the method of the invention is reduced compared to theamount of N₂O produced in a method using a catalyst article comprising awashcoat with the same catalyst composition at the same loading butwithout metal oxide particles dispersed within the small-pore molecularsieve particles. In a still further aspect, the invention provides anexhaust gas treatment system comprising the catalyst composition orcatalyst article according to any embodiment of the present disclosuredownstream from an engine (e.g., a diesel engine or other lean burnengine) and an injector that adds a reductant to the exhaust gas stream.

The invention also provides a method of preparing a catalystcomposition, the method comprising:

dissolving a salt of at least one promoter metal in an aqueous-basedmetal oxide sol, wherein the salt of the at least one promoter metaldissociates in the aqueous-based metal oxide sol to form anaqueous-based metal salt/metal oxide sol mixture, wherein the metaloxide particles comprise one or more oxides of a transition metal orlanthanide of Group 3 or Group 4 of the Periodic Table;

treating ammonium or proton exchanged small-pore molecular sieveparticles having a pore structure and a maximum ring size of eighttetrahedral atoms with the aqueous-based metal salt/metal oxide solmixture to allow impregnation of the promoter metal into the porestructure of the small-pore molecular sieve; and

drying and calcining the treated small-pore molecular sieve particles toform the catalyst composition, wherein the catalyst compositioncomprises the small-pore molecular sieve particles impregnated with thepromoter metal, and metal oxide particles dispersed within thesmall-pore molecular sieve particles and external to the pore structureof the small-pore molecular sieve particles. The promoter metal andmolecular sieve can be selected as described in any embodiment herein.

The metal oxide sol can include any of the metal oxides noted above withrespect to the catalyst composition and can exhibit the same particlesize properties noted above. In certain embodiments, the metal oxide solis selected from the group consisting of zirconyl hydroxide sols,nano-sized hydrous zirconia sols, alumina sols (e.g., large crystal,thermally stable boehmite sols), zirconia-yttria sols, zirconia-aluminasols, zirconia-ceria sols, organo-zirconium sols, and mixtures thereof.Advantageously, the metal oxide particles do not enter the porestructure of the small-pore molecular sieve during the method ofpreparation (i.e., the metal oxide particles are size-excluded from thepore structure of the molecular sieve).

The method can further include the steps of mixing the catalystcomposition with water to form a washcoat slurry; applying the washcoatslurry to a substrate to form a washcoat coating thereon; and drying andcalcining the substrate to form a catalytic article. In certainembodiments, the method includes adding a water soluble metal oxidecompound (e.g., a zirconium compound) to the washcoat slurry to increasethe total metal oxide content thereof.

The invention includes, without limitation, the following embodiments.

Embodiment 1

A catalyst composition suitable for use as a selective catalyticreduction catalyst, comprising: small-pore molecular sieve particleshaving a pore structure and a maximum ring size of eight tetrahedralatoms and impregnated with a promoter metal, and metal oxide particlesdispersed within the small-pore molecular sieve particles and externalto the pore structure of the small-pore molecular sieve particles,wherein the metal oxide particles comprise one or more oxides of atransition metal or lanthanide of Group 3 or Group 4 of the PeriodicTable.

Embodiment 2

The catalyst composition of any preceding or subsequent embodiment,wherein the metal oxide particles comprise a metal oxide selected fromthe group consisting of zirconia, alumina, ceria, hafnia, yttria, andcombinations thereof.

Embodiment 3

The catalyst composition of any preceding or subsequent embodiment,wherein the metal oxide particles comprise zirconia.

Embodiment 4

The catalyst composition of any preceding or subsequent embodiment,wherein the metal oxide particles have an average particle size in therange of about 10 nm to about 500 nm.

Embodiment 5

The catalyst composition of any preceding or subsequent embodiment,wherein the metal oxide particles have a D₁₀ particle size greater thanten times larger than a pore opening of the molecular sieve.

Embodiment 6

The catalyst composition of any preceding or subsequent embodiment,wherein the metal oxide particles have a D₁₀ particle size of about 10nm or greater.

Embodiment 7

The catalyst composition of any preceding or subsequent embodiment,wherein the small-pore molecular sieve has a d6r unit.

Embodiment 8

The catalyst composition of any preceding or subsequent embodiment,wherein the small-pore molecular sieve has a structure type selectedfrom AEI, AFT, AFX, CHA, EAB, ERI, KFI, LEV, LTN, MSO, SAS, SAT, SAV,SFW, and TSC.

Embodiment 9

The catalyst composition of any preceding or subsequent embodiment,wherein the promoter metal is selected from the group consisting of Cu,Co, Ni, La, Mn, Fe, V, Ag, Ce, Nd, Pr, Ti, Cr, Zn, Zn, Nb, Mo, Hf, Y, W,and combinations thereof.

Embodiment 10

The catalyst composition of any preceding or subsequent embodiment,wherein the small-pore molecular sieve has the CHA structure type.

Embodiment 11

The catalyst composition of any preceding or subsequent embodiment,wherein the promoter metal comprises Cu or Fe or combinations thereof.

Embodiment 12

The catalyst composition of any preceding or subsequent embodiment,wherein the promoter metal is present in an amount in the range of about1 to about 10% by weight, based on the total weight of the molecularsieve.

Embodiment 13

The catalyst composition of any preceding or subsequent embodiment,wherein the promoter metal is present in an amount in the range of about2 to about 5% by weight, based on the total weight of the molecularsieve.

Embodiment 14

The catalyst composition of any preceding or subsequent embodiment,wherein the metal oxide is present in an amount in the range of about 1to about 15% by weight, on an oxide basis, based on the total weight ofthe washcoat.

Embodiment 15

A catalyst article comprising a substrate selected from a flow-throughmonolith, a wall-flow filter, a foam, or a mesh, wherein a catalystcomposition according to any preceding or subsequent embodiment isadhered as a washcoat layer on the substrate.

Embodiment 16

The catalyst article of any preceding or subsequent embodiment, whereinthe washcoat is disposed on a flow-through monolith or a wall-flowfilter.

Embodiment 17

The catalyst article of any preceding or subsequent embodiment, whereinthe catalyst article is characterized by an N₂O make that is at least10% by weight lower as compared to a catalyst article comprising awashcoat with the same catalyst composition at the same loading butwithout metal oxide particles dispersed within the small-pore molecularsieve particles.

Embodiment 18

A method for selectively reducing nitrogen oxides (NO_(x)), the methodcomprising contacting an exhaust gas stream containing NO_(x) with thecatalyst article of any preceding or subsequent embodiment.

Embodiment 19

The method of any preceding or subsequent embodiment, wherein the amountof N₂O produced as a byproduct is reduced as compared to the amount ofN₂O produced in a method using a catalyst article comprising a washcoatwith the same catalyst composition at the same loading but without metaloxide particles dispersed within the small-pore molecular sieveparticles.

Embodiment 20

An exhaust gas treatment system comprising the catalyst article of anypreceding or subsequent embodiment downstream from an engine and aninjector that adds a reductant to the exhaust gas stream.

Embodiment 21

A method of preparing a catalyst composition, the method comprising:

dissolving a salt of at least one promoter metal in an aqueous-basedmetal oxide sol, wherein the salt of the at least one promoter metaldissociates in the aqueous-based metal oxide sol to form anaqueous-based metal salt/metal oxide sol mixture, wherein the metaloxide particles comprise one or more oxides of a transition metal orlanthanide of Group 3 or Group 4 of the Periodic Table;

treating ammonium or proton exchanged small-pore molecular sieveparticles having a pore structure and a maximum ring size of eighttetrahedral atoms with the aqueous-based metal salt/metal oxide solmixture to allow impregnation of the promoter metal into the porestructure of the small-pore molecular sieve; and

drying and calcining the treated small-pore molecular sieve particles toform the catalyst composition, wherein the catalyst compositioncomprises the small-pore molecular sieve particles impregnated with thepromoter metal, and metal oxide particles dispersed within thesmall-pore molecular sieve particles and external to the pore structureof the small-pore molecular sieve particles.

Embodiment 22

The method of any preceding or subsequent embodiment, wherein the metaloxide is selected from the group consisting of zirconia, alumina, ceria,hafnia, yttria, and combinations thereof.

Embodiment 23

The method of any preceding or subsequent embodiment, wherein the metaloxide comprises zirconia.

Embodiment 24

The method of any preceding or subsequent embodiment, wherein the metaloxide sol has an average particle size in the range of about 10 nm toabout 500 nm.

Embodiment 25

The method of any preceding or subsequent embodiment, wherein the metaloxide sol has a D₁₀ particle size greater than ten times larger than apore opening of the molecular sieve.

Embodiment 26

The method of any preceding or subsequent embodiment, wherein the metaloxide sol has a D₁₀ particle size of about 10 nm or greater.

Embodiment 27

The method of any preceding or subsequent embodiment, wherein thepromoter metal is selected from the group consisting of Cu, Co, Ni, La,Mn, Fe, V, Ag, Ce, Nd, Pr, Ti, Cr, Zn, Zn, Nb, Mo, Hf, Y, W, andcombinations thereof.

Embodiment 28

The method of any preceding or subsequent embodiment, wherein the metaloxide sol is selected from the group consisting of zirconyl hydroxidesols, nano-sized hydrous zirconia sols, alumina sols, zirconia-yttriasols, zirconia-alumina sols, zirconia-ceria sols, organo-zirconium sols,and mixtures thereof.

Embodiment 29

The method of any preceding or subsequent embodiment, wherein metaloxide particles do not enter the pore structure of the small-poremolecular sieve.

Embodiment 30

The method of any preceding or subsequent embodiment, wherein thesmall-pore molecular sieve has a d6r unit.

Embodiment 31

The method of any preceding or subsequent embodiment, wherein thesmall-pore molecular sieve has a structure type selected from AEI, AFT,AFX, CHA, EAB, ERI, KFI, LEV, LTN, MSO, SAS, SAT, SAV, SFW, and TSC.

Embodiment 32

The method of any preceding or subsequent embodiment, wherein thesmall-pore molecular sieve has the CHA crystal structure.

Embodiment 33

The method of any preceding or subsequent embodiment, wherein thepromoter metal comprises Cu, Fe, or a combination thereof.

Embodiment 34

The method of any preceding or subsequent embodiment, further comprisingthe steps of mixing the catalyst composition with water to form awashcoat slurry; applying the washcoat slurry to a substrate to form awashcoat coating thereon; and drying and calcining the substrate to forma catalytic article.

Embodiment 35

The method of any preceding or subsequent embodiment, further comprisingadding a water soluble metal oxide compound to the washcoat slurry toincrease the total metal oxide content thereof.

These and other features, aspects, and advantages of the disclosure willbe apparent from a reading of the following detailed descriptiontogether with the accompanying drawings, which are briefly describedbelow. The invention includes any combination of two, three, four, ormore of the above-noted embodiments as well as combinations of any two,three, four, or more features or elements set forth in this disclosure,regardless of whether such features or elements are expressly combinedin a specific embodiment description herein. This disclosure is intendedto be read holistically such that any separable features or elements ofthe disclosed invention, in any of its various aspects and embodiments,should be viewed as intended to be combinable unless the context clearlydictates otherwise.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph of the NO_(x) conversion and N₂O make versustemperature of catalyst materials according to the Examples;

FIG. 2 is a SEM image of a catalyst material according to the Examples;

FIG. 3 is a SEM image of a catalyst material according to the Examples;

FIG. 4 is a bar chart of the NO_(x) efficiency for catalyst materialsaccording to the Examples;

FIG. 5 is an SEM image of a catalyst material according to the Examples;

FIGS. 6A-6D are a collection of SEM images of a catalyst materialaccording to the Examples;

FIGS. 7A-7D are a collection of SEM images of a catalyst materialaccording to the Examples;

FIG. 8 is a bar chart of the NO_(x) reduction for catalyst materialsaccording to the Examples;

FIG. 9 is a bar chart of the NH₃ slip, NH₃ storage, and N₂O make forcatalyst materials according to the Examples;

FIGS. 10A-10D is a collection of SEM images of a catalyst materialaccording to the Examples;

FIG. 11 is a graph of the NO_(x) conversion versus temperature ofcatalyst materials according to the Examples;

FIG. 12 is a graph of the N₂O make versus temperature of catalystmaterials according to the Examples;

FIG. 13 is a graph of NO_(x) conversion and N₂O make versus temperatureof a catalyst material according to the Examples in comparison to aprior art material;

FIG. 14 is a graph of NO_(x) conversion versus temperature of catalystmaterials according to the Examples in comparison to a prior artmaterial;

FIG. 15 is a perspective view of a honeycomb-type substrate carrierwhich may comprise a catalyst composition in accordance with the presentinvention; and

FIG. 16 shows a schematic depiction of an embodiment of an emissiontreatment system in which a catalyst composition of the presentinvention is utilized.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

Governmental regulations mandate the use of NO_(x) reductiontechnologies for light and heavy-duty vehicles. Selective catalyticreduction (SCR) of NO_(x) using urea is an effective and dominantemission control technology for NO_(x) control. To meet governmentalregulations, an SCR catalyst that has improved performance compared tothe current Cu-SSZ-13 based benchmark technology is necessary. Providedis a catalyst having improved NO_(x) conversion efficiency and lower N₂Omake relative to the current Cu-SSZ-13 based benchmark technologies incertain embodiments. The catalyst effectively promotes the reaction ofammonia with nitrogen oxides to form nitrogen and H₂O selectively over atemperature range of 200 to 600° C.

Embodiments of the invention are directed to a selective catalyticreduction catalyst including a small-pore molecular sieve and a zirconiacontaining layer. Surprisingly, it was found that modification of asmall-pore molecular sieve with zirconia resulted in lower N₂O make andan improved low to high temperature performance window. In certainembodiments, the present invention provides a catalyst composition inthe form of an intimate mixture of small-pore molecular sieve particlesimpregnated with a promoter metal and metal oxide (e.g., zirconia)particles. The metal oxide particles are sized so as to prevent anysignificant metal oxide particle penetration into the pore structure ofthe molecular sieve. Instead, the metal oxide particles essentiallyprovide a surface coating on the molecular sieve particles. The presenceof the metal oxide particles has been found to improve low temperatureNO_(x) reduction and reduce N₂O make.

With respect to the terms used in this disclosure, the followingdefinitions are provided.

As used herein, the term “catalyst” or “catalyst composition” or“catalyst material” refers to a material that promotes a reaction.

As used herein, the term “catalytic article” refers to an element thatis used to promote a desired reaction. For example, a catalytic articlemay comprise a washcoat containing a catalytic species, e.g. a catalystcomposition, on a substrate.

As used herein, the term “selective catalytic reduction” (SCR) refers tothe catalytic process of reducing oxides of nitrogen to dinitrogen (N₂)using a nitrogenous reductant.

As used herein, the term “washcoat” has its usual meaning in the art ofa thin, adherent coating of a catalytic or other material applied to acarrier substrate material, such as a honeycomb-type carrier member,which is sufficiently porous to permit the passage of the gas streambeing treated. As is understood in the art, a washcoat is obtained froma dispersion of particles in a slurry, which is applied to a substrate,dried and calcined to provide the porous washcoat.

In one or more embodiments, a selective catalytic reduction catalystcomprises a washcoat including a small-pore molecular sieve having apore structure and a maximum ring size of eight tetrahedral atoms andcontaining a promoter metal, and a zirconia containing layer on thesmall-pore molecular sieve containing the promoter metal, wherein thezirconia containing layer advantageously has particles of zirconia witha particle size in the range of about 10 nm to about 500 nm.

Molecular Sieves

As used herein, the phrase “molecular sieve” refers to frameworkmaterials such as zeolites and other framework materials (e.g.isomorphously substituted materials), which may in particulate form, andin combination with one or more promoter metals, be used as catalysts.Molecular sieves are materials based on an extensive three-dimensionalnetwork of oxygen ions containing generally tetrahedral type sites andhaving a substantially uniform pore distribution, with the average poresize being no larger than 20 Å. The pore sizes are defined by the ringsize. As used herein, the term “zeolite” refers to a specific example ofa molecular sieve, including silicon and aluminum atoms. According toone or more embodiments, it will be appreciated that by defining themolecular sieves by their structure type, it is intended to include thestructure type and any and all isotypic framework materials such asSAPO, ALPO, and MeAPO materials having the same structure type as thezeolite materials.

In more specific embodiments, reference to an aluminosilicate zeolitestructure type limits the material to molecular sieves that do notinclude phosphorus or other metals substituted in the framework.However, to be clear, as used herein, “aluminosilicate zeolite” excludesaluminophosphate materials such as SAPO, ALPO, and MeAPO materials, andthe broader term “zeolite” is intended to include aluminosilicates andaluminophosphates. Zeolites are crystalline materials having ratheruniform pore sizes which, depending upon the type of zeolite and thetype and amount of cations included in the zeolite lattice, range fromabout 3 to 10 Angstroms in diameter. Zeolites generally comprise silicato alumina (SAR) molar ratios of 2 or greater.

The term “aluminophosphates” refers to another specific example of amolecular sieve, including aluminum and phosphate atoms.Aluminophosphates are crystalline materials having rather uniform poresizes.

Aluminosilicates generally comprise open 3-dimensional frameworkstructures composed of corner-sharing TO₄ tetrahedra, where T is Al orSi, or optionally P. Cations that balance the charge of the anionicframework are loosely associated with the framework oxygens, and theremaining pore volume is filled with water molecules. The non-frameworkcations are generally exchangeable, and the water molecules removable.

In one or more embodiments, the small-pore molecular sieve comprisesSiO₄/AlO₄ tetrahedra and is linked by common oxygen atoms to form athree-dimensional network. In other embodiments, the molecular sievecomponent comprises SiO₄/AlO₄/PO₄ tetrahedra. The small-pore molecularsieve of one or more embodiments is differentiated mainly according tothe geometry of the voids which are formed by the rigid network of the(SiO₄)/AlO₄, or SiO₄/AlO₄/PO₄, tetrahedra. The entrances to the voidsare formed from 6, 8, 10, or 12 ring atoms with respect to the atomswhich form the entrance opening. In one or more embodiments, themolecular sieve comprises ring sizes of no larger than 8, including 6and 8.

According to one or more embodiments, the molecular sieve can be basedon the framework topology by which the structures are identified.Typically, any structure type of zeolite can be used, such as structuretypes of ABW, ACO, AEI, AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS,AFT, AFX, AFY, AHT, ANA, APC, APD, AST, ASV, ATN, ATO, ATS, ATT, ATV,AWO, AWW, BCT, BEA, BEC, BIK, BOG, BPH, BRE, CAN, CAS, SCO, CFI, SGF,CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO, DFT, DOH, DON, EAB, EDI,EMT, EON, EPI, ERI, ESV, ETR, EUO, FAU, FER, FRA, GIS, GIU, GME, GON,GOO, HEU, IFR, IHW, ISV, ITE, ITH, ITW, IWR, IWW, JBW, KFI, LAU, LEV,LIO, LIT, LOS, LOV, LTA, LTL, LTN, MAR, MAZ, MEI, MEL, MEP, MER, MFI,MFS, MON, MOR, MOZ, MSO, MTF, MTN, MTT, MTW, MWW, NAB, NAT, NES, NON,NPO, NSI, OBW, OFF, OSI, OSO, OWE, PAR, PAU, PHI, PON, RHO, RON, RRO,RSN, RTE, RTH, RUT, RWR, RWY, SAO, SAS, SAT, SAV, SBE, SBS, SBT, SFE,SFF, SFG, SFH, SFN, SFO, SGT, SOD, SOS, SSY, STF, STI, STT, TER, THO,TON, TSC, UEI, UFI, UOZ, USI, UTL, VET, VFI, VNI, VSV, WIE, WEN, YUG,ZON, or combinations thereof.

In one or more embodiments, the molecular sieve comprises an 8-ringsmall pore aluminosilicate zeolite. As used herein, “small pore” refersto pore openings which are smaller than about 5 Angstroms, for exampleon the order of ˜3.8 Angstroms. The phrase “8-ring” zeolites refers tozeolites having 8-ring pore openings and double-six ring secondarybuilding units and having a cage like structure resulting from theconnection of double six-ring building units by 4 rings. Zeolites arecomprised of secondary building units (SBU) and composite building units(CBU), and appear in many different framework structures. Secondarybuilding units contain up to 16 tetrahedral atoms and are non-chiral.Composite building units are not required to be achiral, and cannotnecessarily be used to build the entire framework. For example, a groupof zeolites have a single 4-ring (s4r) composite building unit in theirframework structure. In the 4-ring, the “4” denotes the positions oftetrahedral silicon and aluminum atoms, and the oxygen atoms are locatedin between tetrahedral atoms. Other composite building units include,for example, a single 6-ring (s6r) unit, a double 4-ring (d4r) unit, anda double 6-ring (d6r) unit. The d4r unit is created by joining two s4runits. The d6r unit is created by joining two s6r units. In a d6r unit,there are twelve tetrahedral atoms. Zeolitic structure types that have ad6r secondary building unit include AEI, AFT, AFX, CHA, EAB, EMT, ERI,FAU, GME, JSR, KFI, LEV, LTL, LTN, MOZ, MSO, MWW, OFF, SAS, SAT, SAV,SBS, SBT, SFW, SSF, SZR, TSC, and WEN.

In one or more embodiments, the molecular sieve is a small-poremolecular sieve having a pore structure and a maximum ring size of eighttetrahedral atoms. In other embodiments, the small-pore molecular sievecomprises a d6r unit. Thus, in one or more embodiments, the small-poremolecular sieve has a structure type selected from AEI, AFT, AFX, CHA,EAB, ERI, KFI, LEV, LTN, MSO, SAS, SAT, SAV, SFW, TSC, and combinationsthereof. In other specific embodiments, the molecular sieve has astructure type selected from the group consisting of CHA, AEI, AFX, ERI,KFI, LEV, and combinations thereof. In still further specificembodiments, the small-pore molecular sieve has a structure typeselected from CHA, AEI, and AFX. In one or more very specificembodiments, the small-pore molecular sieve component has the CHAstructure type.

Zeolitic chabazite includes a naturally occurring tectosilicate mineralof a zeolite group with approximate formula: (Ca,Na₂,K₂,Mg)Al₂Si₄O₁₂.6H₂O (e.g., hydrated calcium aluminum silicate). Three synthetic forms ofzeolitic chabazite are described in “Zeolite Molecular Sieves,” by D. W.Breck, published in 1973 by John Wiley & Sons, which is herebyincorporated by reference. The three synthetic forms reported by Breckare Zeolite K-G, described in J. Chem. Soc., p. 2822 (1956), Barrer etal; Zeolite D, described in British Patent No. 868,846 (1961); andZeolite R, described in U.S. Pat. No. 3,030,181, which are herebyincorporated by reference. Synthesis of another synthetic form ofzeolitic chabazite, SSZ-13, is described in U.S. Pat. No. 4,544,538,which is hereby incorporated by reference. Synthesis of a synthetic formof a molecular sieve having the chabazite crystal structure,silicoaluminophosphate 34 (SAPO-34), is described in U.S. Pat. Nos.4,440,871 and 7,264,789, which are hereby incorporated by reference. Amethod of making yet another synthetic molecular sieve having chabazitestructure, SAPO-44, is described in U.S. Pat. No. 6,162,415, which ishereby incorporated by reference.

In one or more embodiments, the molecular sieve can include allaluminosilicate, borosilicate, gallosilicate, MeAPSO, and MeAPOcompositions. These include, but are not limited to SSZ-13, SSZ-62,natural chabazite, zeolite K-G, Linde D, Linde R, LZ-218, LZ-235.LZ-236, ZK-14, SAPO-34, SAPO-44, SAPO-47, ZYT-6, CuSAPO-34, CuSAPO-44,and CuSAPO-47.

The ratio of silica to alumina of an aluminosilicate molecular sieve canvary over a wide range. In one or more embodiments, the molecular sievehas a silica to alumina molar ratio (SAR) in the range of 2 to 300,including 5 to 250; 5 to 200; 5 to 100; and 5 to 50. In one or morespecific embodiments, the molecular sieve has a silica to alumina molarratio (SAR) in the range of 10 to 200, 10 to 100, 10 to 75, 10 to 60,and 10 to 50; 15 to 100, 15 to 75, 15 to 60, and 15 to 50; 20 to 100, 20to 75, 20 to 60, and 20 to 50.

In one or more specific embodiments, the small-pore molecular sieve hasthe CHA structure type and has a silica-to-alumina ratio of from 2 to300, including 5 to 250, 5 to 200, 5 to 100, and 5 to 50; 10 to 200, 10to 100, 10 to 75, 10 to 60, and 10 to 50; 15 to 100, 15 to 75, 15 to 60,and 15 to 50; 20 to 100, 20 to 75, 20 to 60, and 20 to 50. In a specificembodiment, the small-pore molecular sieve comprises SSZ-13. In a veryspecific embodiment, the SSZ-13 has a silica-to-alumina ratio of from 2to 300, including 5 to 250, 5 to 200, 5 to 100, and 5 to 50; 10 to 200,10 to 100, 10 to 75, 10 to 60, and 10 to 50; 15 to 100, 15 to 75, 15 to60, and 15 to 50; 20 to 100, 20 to 75, 20 to 60, and 20 to 50.

Synthesis of zeolites and related micro- and mesoporous materials variesaccording to the structure type of the zeolitic material, but typicallyinvolves the combination of several components (e.g. silica, alumina,phosphorous, alkali, organic template etc.) to form a synthesis gel,which is then hydrothermally crystallized to form a final product. Thestructure directing agent (SDA) can be in the form of an organic, i.e.,tetraethylammonium hydroxide (TEAOH), or inorganic cation, i.e. Na⁺ orK⁺. During crystallization, the tetrahedral units organize around theSDA to form the desired framework, and the SDA is often embedded withinthe pore structure of the zeolite crystals. In one or more embodiments,the crystallization of the molecular sieve materials can be obtained bymeans of the addition of structure-directing agents/templates, crystalnuclei or elements. In some instances, the crystallization can beperformed at temperatures of less than 100° C. A molecular sieve havingthe CHA structure may be prepared according to various techniques knownin the art, for example U.S. Pat. No. 4,544,538 (Zones) and U.S. Pat.No. 6,709,644 (Zones), which are herein incorporated by reference intheir entireties.

Optionally, the obtained alkali metal zeolite is NH₄-exchanged to formNH₄—Chabazite. The NH₄-ion exchange can be carried out according tovarious techniques known in the art, for example Bleken, F.; Bjorgen,M.; Palumbo, L.; Bordiga, S.; Svelle, S.; Lillerud, K.-P.; and Olsbye,U. Topics in Catalysis 52, (2009), 218-228.

Promoter Metal

As used herein, “promoted” refers to a component that is intentionallyadded to the molecular sieve, as opposed to impurities inherent in themolecular sieve. Thus, a promoter is intentionally added to enhanceactivity of a catalyst compared to a catalyst that does not havepromoter intentionally added. In order to promote the SCR of oxides ofnitrogen, in one or more embodiments, a suitable metal is exchanged intothe molecular sieve component. Accordingly, the molecular sieve of oneor more embodiments may be subsequently ion-exchanged with one or morepromoter metals such as copper (Cu), cobalt (Co), nickel (Ni), lanthanum(La), manganese (Mn), iron (Fe), vanadium (V), silver (Ag), and cerium(Ce), neodymium (Nd), praseodymium (Pr), titanium (Ti), chromium (Cr),zinc (Zn), tin (Sn), niobium (Nb), molybdenum (Mo), hafnium (Hf),yttrium (Y), and tungsten (W). In specific embodiments, the molecularsieve component is promoted with Cu, Fe, and combinations thereof. Invery specific embodiments, the molecular sieve component is promotedwith Cu.

The promoter metal content of the molecular sieve component, calculatedas the oxide, is, in one or more embodiments, at least about 0.1 wt. %,reported on a volatile-free basis. In one or more embodiments, thepromoter metal is present in an amount in the range of about 1 to about10% by weight, including the range of about 2 to about 5% by weight, inall cases, based on the total weight of the molecular sieve. In one ormore specific embodiments, the promoter metal comprises Cu, and the Cucontent, calculated as CuO is in the range of up to about 10 wt. %,including 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.5, and 0.1 wt. %, on an oxidebasis, in each case based on the total weight of the calcined molecularsieve component and reported on a volatile free basis. In specificembodiments, the Cu content, calculated as CuO, is in the range of about2 to about 5 wt. %.

Metal Oxide on the Surface of the Molecular Sieve

According to one or more embodiments, the catalyst composition includesmolecular sieve contains a promoter metal and a metal oxide on thesurface of the molecular sieve. The metal oxide is in intimate mixturewith the molecular sieve so as to provide a dispersed metal oxide phasewithin the molecular sieve material. In certain embodiments, thedispersion of the metal oxide throughout the molecular sieve materialcan be relatively uniform. However, in some embodiments, at least aportion of the metal oxide can be found in an enriched region at thesurface of a washcoat layer containing the catalyst composition of theinvention, usually as a result of an amount of water soluble zirconiumcompound (or other metal oxide compound) that migrates to the washcoatlayer surface and is decomposed/oxidized in air during the substratecoating process.

For the sake of ease of reference, much of the present disclosurefocuses on zirconium oxide (and related zirconium precursors). However,other metal oxides can be used without departing from the invention,such as metal oxides comprising one or more oxides of a transition metalor lanthanide of Group 3 or Group 4 of the Periodic Table. Specificexamples include zirconia, alumina, ceria, hafnia, yttria, andcombinations thereof, although minor amounts of other metal oxides couldalso be present. In certain embodiments, the predominant (greater than50% by weight based on total metal oxide weight) metal oxide iszirconia, alumina, ceria, hafnia, yttria, or a combination thereof. Incertain advantageous embodiments, the metal oxide is predominantlyzirconia, including composites of zirconia with other metal oxides suchas ceria, alumina, hafnia, or yttria. In other embodiments, the metaloxide is alumina, such as large crystal boehmite materials such asboehmite having a crystallite size of about 20 nm or higher.

In certain embodiments, the metal oxide content of the catalystcomposition is provided, at least in part, by the mixture of a metaloxide sol containing microparticles or nanoparticles of the metal oxidewith the molecular sieve. Introduction of the metal oxide material in arelatively insoluble form, as opposed to use of water soluble precursorsthat are later calcined into oxide form, can aid in prevention ofpromoter metal migration within the catalyst composition, which can bedetrimental to high temperature NO_(x) reduction. Thus, the benefits ofincreased metal oxide content (reduced N₂O make and enhanced lowtemperature NO_(x) reduction) can be achieved without negativelyimpacting high temperature performance.

For example, in some embodiments, the zirconia is introduced using anaqueous-based zirconia sol. As used herein, the term “aqueous-basedzirconia sol” refers to a colloidal suspension of small, solid particlesof zirconia or hydrous zirconia in a continuous liquid (water) medium.In one or more embodiments, the aqueous-based zirconia sol is selectedfrom the group consisting of zirconyl hydroxide sols, nano-sized hydrouszirconia sols, zirconia-yttria sols, zirconia-alumina sols,zirconia-ceria sol, organo-zirconium sols, and mixtures thereof. Asnoted herein, an aqueous-based alumina sol, such as a large crystalboehmite sol, could also be used in certain embodiments. In someembodiments, the aqueous-based zirconia sol may include one or morepromoter metals in the form of an aqueous-based metal salt. In otherwords, the molecular sieve can be impregnated with a promoter metal andmixed with metal oxide particles in the same treatment step. As usedherein, “promoted” refers to a component that is intentionally added tothe aqueous-based zirconia sol, as opposed to impurities inherent in theaqueous-based zirconia sol. Thus, a promoter is intentionally added toenhance activity of the aqueous-based zirconia sol compared to anaqueous-based zirconia sol that does not have promoter intentionallyadded. In one or more embodiments, the aqueous-based zirconia solincludes a promoter metal selected from the group consisting oflanthanum (La), cerium (Ce), neodymium (Nd), praseodymium (Pr), copper(Cu), manganese (Mn), iron (Fe), nickel (Ni), titanium (Ti), chromium(Cr), zinc (Zn), tin (Sn), vanadium (V), niobium (Nb), molybdenum (Mo),hafnium (Hf), tungsten (W), yttrium (Y), and combinations thereof.

In one or more embodiments, the zirconium sol (or other metal oxide sol)referenced herein has particles of zirconia (or other metal oxide) withan average particle size in the range of about 10 nm to about 500 nm,including about 10 nm to about 400 nm, about 10 nm to about 300 nm, andabout 10 nm to about 250 nm. As used herein, the term “average particlesize” refers to the average diameter of the zirconia particles (or othermetal oxide particles) as measured by CILAS 1064 Laser Particle SizeAnalyzer, according to the manufacturer's recommended liquid mode methodwith a measurement range of 0.04 to 500 microns. The particle sizes ofnano-sized sol component can be measured using CILAS 1064 Laser ParticleSize Analyzer according to the manufacturer's recommended liquid modemethod with a measurement range of 0.04 to 500 microns. For particles<40 nm, such particle sizes can be measured using Malvern Zetasizer NanoZS, which is a high performance two angle particle and molecular sizeanalyzer for the enhanced detection of aggregates and measurement ofsmall or dilute samples, and samples at very low or high concentrationusing dynamic light scattering with “NIBS” optics.

In one or more embodiments, the molecular sieve and the particles ofzirconia (or other metal oxide) have an average or mean particle sizedistribution ratio of greater than about 10:1, including greater thanabout 100:1, greater than about 1000:1, greater than about 10,000:1. Asused herein, the terms “average particle size distribution ratio” and“mean particle size distribution ratio” refer to D₅₀ (50%=value).

Without intending to be bound by theory, it is thought that the zirconia(or other metal oxide) should advantageously contain nano-sizedparticles, which are sized such that the D₁₀ value is greater than tentimes (10×) that of the pore opening of the zeolite such that theparticles will not penetrate the pores of the small pore molecularsieve. In one or more embodiments, the particles of zirconia (and/or theparticles of zirconia in the starting zirconia sol) have a D₁₀ particlesize that is more than ten times larger than a pore opening of thesmall-pore molecular sieve. Reference to D₁₀ particle size means aparticle distribution having 10% by weight of particles with a diameterbelow the given threshold. In certain embodiments, the particles ofzirconia have a D₁₀ value of about 10 nm or greater, about 15 nm orgreater, or about 20 nm or greater.

Surprisingly, it was found that the presence of zirconia reduces N₂Omake. In one or more embodiments, for certain catalyst articles of theinvention, N₂O make is reduced by more than about 10% by weight,including more than about 15%, more than about 20%, more than about 25%,more than about 30%, more than about 35%, and more than about 40%, whencompared to a catalyst article comprising a washcoat including the samesmall-pore molecular sieve/promoter metal (at the same catalyst andpromoter metal loading), but which does not comprise a zirconiacontaining layer. Exemplary test conditions for determining N₂O make canbe found in Example 3.

In one or more embodiments, the zirconia (or other metal oxide) ispresent in an amount in a range of about 1 to about 20% by weight,including about 1%, about 2%, about 3%, about 4%, about 5%, about 6%,about 7%, about 8%, about 9%, about 10%, about 11%, about 12%, about13%, about 14%, and about 15% by weight, on an oxide basis, in all casesbased on the total weight of the washcoat. As used herein, the term“total weight of the washcoat” means the weight of all of the componentsin the washcoat, including the molecular sieve, the promoter metal, andthe zirconia, after the washcoat has been dried and calcined. In certainembodiments, the zirconia (or other metal oxide) is present in an amountof at least about 5% by weight, at least about 6%, at least about 7%, atleast about 8%, at least about 9%, or at least about 10%, based on thetotal weight of the washcoat.

In certain embodiments, the metal oxide content is supplemented throughuse of a water soluble metal oxide precursor added to the washcoatslurry during the washcoating process. In one embodiment, the entiretyof the metal oxide is provided in this manner, although it isadvantageous for at least some of the metal oxide content to be derivedfrom a relatively insoluble source as described above. As noted in theexamples, use of large amounts of water soluble metal oxide precursormay contribute to undesirable promoter metal migration within awashcoat. Accordingly, it is desirable to either minimize thecontribution of water soluble metal oxide precursors to the overallmetal oxide content and/or calcine the metal-promoted molecular sievematerial (e.g., calcination in air at a temperature of at least about300° C.) before contact with the water soluble metal oxide precursor tominimize promoter metal solubility.

A washcoat slurry is typically prepared using copious amounts of water,and, thus, an aqueous based washcoat slurry is typically used as well.The zirconium compounds that migrate to the washcoat surface are solublein water and must also then be soluble in the slurry. In one or moreembodiments, the zirconium compound is at least 15% by weight soluble inwater, including at least about 20% soluble, at least about 30% soluble,at least about 40% soluble, at least about 50% soluble, at least about60% soluble, at least about 70% soluble, at least about 80% soluble, andat least about 90% soluble. In other embodiments, the zirconium compoundhas a water solubility in the range of about 15 to about 100% soluble,including about 15 to about 85%, about 20 to about 100%, about 20 toabout 85%, about 30 to about 100%, about 30 to about 85%, about 40 toabout 100%, about 40 to about 85%, about 50 to about 100%, and about 50to about 85% soluble. Reference to solubility in terms of weightpercentage refers to the percentage of zirconium compound dissolved inan aqueous washcoat composition at room temperature (25° C.) and 1 atm.

As used herein, “water soluble zirconia component,” “water solublezirconium compound,” and the like refers to the respective water solublezirconium-containing compound, complex, precursor, or the like which,upon calcination or use of the catalyst, decomposes, oxidizes, orotherwise converts to a catalytically active form, usually, the metal orthe metal oxide (i.e., zirconia). As noted above, water solubleprecursors of other metal oxides could be used instead of zirconiumcompounds.

In one or more embodiments, a salt, such as e.g. NH₄NO₃ or NH₄OAc, isadded to the aqueous washcoat composition containing the zirconiumcompound in order to increase ionic strength. The pH is thenadjusted/controlled, e.g. pH ˜4-5, in order to ensure the zirconiumcompound such as a zirconyl salt, e.g. zirconyl acetate, is watersoluble and will migrate during drying.

In one or more embodiments, the zirconium compound is selected from thegroup consisting of ionic zirconium salts, covalently bondedorgano-zirconium complexes, covalently bonded organo-zirconyl compounds,and mixtures thereof. As used herein, there term “organo-zirconium salt,compound, or complex” refers to Zr⁴⁺ with any anionic organic ligandcovalently bonded to form a complex, and which can also includepolymeric species. In one or more embodiments, the water solublezirconium compound is selected from the group consisting of zirconiumacetate, zirconium citrate, zirconium tartrate, zirconium lactate,zirconium adipate, and mixtures thereof.

As used herein, the term “organo-zirconyl salt, compound, or complex”refers to ZrO²⁺ with any anionic organic ligand ionically bonded to forma complex, which can also include polymeric species. In one or moreembodiments, the water soluble zirconium compound is selected from thegroup consisting of zirconium nitrate, zirconium chloride, zirconiumsulfate, zirconyl nitrate, zirconyl chloride, zirconyl sulfate, zirconylacetate, zirconyl citrate, zirconyl tartrate, zirconyl lactate, zirconyladipate, and mixtures thereof.

Particle Shape and Size

The catalyst according to embodiments of the invention may be providedin the form of a powder or a sprayed material from separation techniquesincluding decantation, filtration, centrifugation, or spraying.

In general, the powder or sprayed material can be shaped without anyother compounds, e.g. by suitable compacting, to obtain moldings of adesired geometry, e.g. tablets, cylinders, spheres, or the like.

By way of example, the powder or sprayed material is admixed with orcoated by suitable modifiers well known in the art. By way of example,modifiers such as silica, alumina, zeolites or refractory binders (forexample a zirconium precursor) may be used. The powder or the sprayedmaterial, optionally after admixing or coating by suitable modifiers,may be formed into a slurry, for example with water, which is depositedupon a suitable refractory carrier, for example, a flow throughhoneycomb substrate carrier or a wall flow honeycomb substrate carrier.

The catalyst according to embodiments of the invention may also beprovided in the form of extrudates, pellets, tablets, or particles ofany other suitable shape, for use as a packed bed of particulatecatalyst, or as shaped pieces such as plates, saddles, tubes, or thelike.

SCR Activity

In one or more embodiments, a coated substrate comprising the selectivecatalytic reduction catalyst of one or more embodiments exhibits an agedNO_(x) conversion at 200° C. of at least 50% measured at a gas hourlyspace velocity of 80000 h⁻¹. In specific embodiments the catalystexhibits an aged NO_(x) conversion at 450° C. of at least 70% measuredat a gas hourly space velocity of 80000 h⁻¹. More specifically the agedNO_(x) conversion at 200° C. is at least 55% and at 450° C. at least75%, even more specifically the aged NO_(x) conversion at 200° C. is atleast 60% and at 450° C. at least 80%, measured at a gas hourlyvolume-based space velocity of 80000 h⁻¹ under steady state conditionsat maximum NH₃-slip conditions in a gas mixture of 500 ppm NO, 500 ppmNH₃, 10% O₂, 5% H₂O, balance N₂. The coated substrates or “cores” werehydrothermally aged in a tube furnace in a gas flow containing 10% H₂O,10% O₂, balance N₂ at a space velocity of 4,000 h⁻¹ for 5 h at 750° C.

The SCR activity measurement has been demonstrated in the literature,see, for example PCT Application Publication No. WO 2008/106519.

Furthermore, according to one or more embodiments, the catalyst iseffective to lower N₂O make.

The Substrate

In one or more embodiments, the catalyst composition can be applied to asubstrate as a washcoat. As used herein, the term “substrate” refers tothe monolithic material onto which the catalyst is placed, typically inthe form of a washcoat. A washcoat is formed by preparing a slurrycontaining a certain solids content (e.g., 30-90% by weight) of catalystin a liquid vehicle, which is then coated onto a substrate and dried toprovide a washcoat layer.

In one or more embodiments, the substrate is selected from one or moreof a flow-through honeycomb monolith, a wall-flow filter, a foam, or amesh, and the catalyst is applied to the substrate as a washcoat.

According to one or more embodiments, the substrate for the catalystcomposition may be constructed of any material typically used forpreparing automotive catalysts and will typically comprise a metal orceramic honeycomb structure. The substrate typically provides aplurality of wall surfaces upon which the catalyst composition isapplied and adhered, thereby acting as a carrier for the catalystcomposition.

Exemplary metallic substrates include heat resistant metals and metalalloys, such as titanium and stainless steel as well as other alloys inwhich iron is a substantial or major component. Such alloys may containone or more of nickel, chromium, and/or aluminum, and the total amountof these metals may advantageously comprise at least 15 wt. % of thealloy, e.g., 10-25 wt. % of chromium, 3-8 wt. % of aluminum, and up to20 wt. % of nickel. The alloys may also contain small or trace amountsof one or more other metals, such as manganese, copper, vanadium,titanium and the like. The surface or the metal carriers may be oxidizedat high temperatures, e.g., 1000° C. and higher, to form an oxide layeron the surface of the substrate, improving the corrosion resistance ofthe alloy and facilitating adhesion of the washcoat layer to the metalsurface.

Ceramic materials used to construct the substrate may include anysuitable refractory material, e.g., cordierite, mullite, cordierite-αalumina, silicon nitride, zircon mullite, spodumene, alumina-silicamagnesia, zircon silicate, sillimanite, magnesium silicates, zircon,petalite, a alumina, aluminosilicates and the like.

Any suitable substrate may be employed, such as a monolithicflow-through substrate having a plurality of fine, parallel gas flowpassages extending from an inlet to an outlet face of the substrate suchthat passages are open to fluid flow. The passages, which areessentially straight paths from the inlet to the outlet, are defined bywalls on which the catalytic material is coated as a washcoat so thatthe gases flowing through the passages contact the catalytic material.The flow passages of the monolithic substrate are thin-walled channelswhich can be of any suitable cross-sectional shape, such as trapezoidal,rectangular, square, sinusoidal, hexagonal, oval, circular, and thelike. Such structures may contain from about 60 to about 1200 or moregas inlet openings (i.e., “cells”) per square inch of cross section(cpsi), more usually from about 300 to 600 cpsi. The wall thickness offlow-through substrates can vary, with a typical range being between0.002 and 0.1 inches. A representative commercially-availableflow-through substrate is a cordierite substrate having 400 cpsi and awall thickness of 6 mil, or 600 cpsi and a wall thickness of 4 mil.However, it will be understood that the invention is not limited to aparticular substrate type, material, or geometry.

In alternative embodiments, the substrate may be a wall-flow substrate,wherein each passage is blocked at one end of the substrate body with anon-porous plug, with alternate passages blocked at opposite end-faces.This requires that gas flow through the porous walls of the wall-flowsubstrate to reach the exit. Such monolithic substrates may contain upto about 700 or more cpsi, such as about 100 to 400 cpsi and moretypically about 200 to about 300 cpsi. The cross-sectional shape of thecells can vary as described above. Wall-flow substrates typically have awall thickness between 0.002 and 0.1 inches. A representativecommercially available wall-flow substrate is constructed from a porouscordierite, an example of which has 200 cpsi and 10 mil wall thicknessor 300 cpsi with 8 mil wall thickness, and wall porosity between 45-65%.Other ceramic materials such as aluminum-titanate, silicon carbide andsilicon nitride are also used a wall-flow filter substrates. However, itwill be understood that the invention is not limited to a particularsubstrate type, material, or geometry. Note that where the substrate isa wall-flow substrate, the DOC composition can permeate into the porestructure of the porous walls (i.e., partially or fully occluding thepore openings) in addition to being disposed on the surface of thewalls.

FIG. 15 illustrates an exemplary substrate 2 in the form of a honeycombmonolith coated with a catalyst composition as described herein. Theexemplary substrate 2 has a cylindrical shape and a cylindrical outersurface 4, an upstream end face 6 and a corresponding downstream endface 8, which is identical to end face 6. Substrate 2 has a plurality offine, parallel gas flow passages 10 formed therein. In the case of aflow-through monolith, the passages 10 are typically unobstructed so asto permit the flow of a fluid, e.g., a gas stream, longitudinallythrough carrier 2 via gas flow passages 10 thereof. Alternatively, thesubstrate 2 can be in the form of a wall-flow filter as discussed indetail above. In such an embodiment, each gas flow passage 10 is blockedat either the inlet or outlet end and the walls of the passages areporous to allow gas to travel from one gas flow passage into an adjacentgas flow passage, as would be understood in the art. If desired, thecatalyst composition can be applied in multiple, distinct layers. Thepresent invention can be practiced with one or more (e.g., 2, 3, or 4)washcoat layers.

To coat the substrates with the catalyst of one or more embodiments, thesubstrates are immersed vertically in a portion of the catalyst slurrysuch that the top of the substrate is located just above the surface ofthe slurry. In this manner slurry contacts the inlet face of eachhoneycomb wall, but is prevented from contacting the outlet face of eachwall. The sample is left in the slurry for about 30 seconds. Thesubstrate is removed from the slurry, and excess slurry is removed fromthe substrate first by allowing it to drain from the channels, then byblowing with compressed air (against the direction of slurrypenetration), and then by pulling a vacuum from the direction of slurrypenetration. By using this technique, in the case of a wall-flowsubstrate, the catalyst slurry permeates the walls of the substrate, yetthe pores are not occluded to the extent that undue back pressure willbuild up in the finished substrate. As used herein, the term “permeate”when used to describe the dispersion of the catalyst slurry on thesubstrate, means that the catalyst composition is dispersed throughoutthe wall of the substrate and, thus, at least partially occlude thepores in the wall.

The coated substrates are dried typically at about 100° C. and calcinedat a higher temperature (e.g., 300 to 450° C.). After calcining, thecatalyst loading can be determined through calculation of the coated anduncoated weights of the substrate. As will be apparent to those of skillin the art, the catalyst loading can be modified by altering the solidscontent of the coating slurry. Alternatively, repeated immersions of thesubstrate in the coating slurry can be conducted, followed by removal ofthe excess slurry as described above.

Preparation of Catalyst

According to one or more embodiments, methods for the synthesis ofselective catalytic reduction catalysts are provided. More particularly,the catalyst comprises a small-pore molecular sieve having a porestructure and a maximum ring size of eight tetrahedral atoms andcontaining a promoter metal, and a zirconia containing layer on thesmall-pore molecular sieve containing the promoter metal, wherein thezirconia containing layer typically has particles of zirconia with anaverage particle size in the range of about 10 nm to about 500 nm.

For ease of reference, the foregoing description focuses on the use of azirconia sol. However, it is understood that other metal oxide solscould be used without departing from the invention. In one embodiment,the catalyst can be prepared by dissolving a metal salt (e.g., a nitrateor acetate salt) in an aqueous-based zirconia sol, such as, but notlimited to, colloidal zirconyl hydroxide. In one or more embodiments,the metal salt is a salt of at least one metal selected from the groupconsisting of La, Ce, Nd, Pr, Cu, Mn, Fe, Ni, Ti, Cr, Zn, Sn, V, Nb, Mo,Hf, Y, and W. The metal salt dissolves and dissociates in theaqueous-based zirconia sol to form a soluble aqueous-based metalsalt/zirconia sol mixture. Exemplary metal salts include copper (II)nitrate, copper (II) acetate, iron (III) nitrate, and iron (III)acetate. The aqueous-based zirconia sol has particles of zirconia withan average particle size in the range of about 10 nm to about 500 nm. Anaqueous-based metal salt (i.e. promoter metal salt)/zirconia sol mixtureis formed, having a concentration of about 50 to 100% incipient wetness.In one or more embodiments, a higher incipient wetness, particularlyapproaching 100% is desired.

Subsequently, an ammonium or proton exchanged molecular sieve isimpregnated with the metal salt/zirconia-based sol mixture. Impregnationcan occur in any of various mixers known in the art for mixing a powderwith a solution or a dispersion, such as a ribbon mixer or a planetarymixer equipped with a nozzle for spraying a liquid into the mixer. Theimpregnated material is dried and calcined in air, forming the catalyst,which comprises a metal exchanged/promoted molecular sieve having azirconia containing layer. Calcination of the impregnated material canoccur using various techniques known in the art, including traycalcination methods, calcination in a rotary kiln, or through use of afluidized bed calciner. Flash drying and calcination in a single step(e.g., using a fluidized bed calciner) is preferred in certainembodiments as such methods provide short residence times and uniformdrying/calcination on a particle level.

Without intending to be bound by theory, it is thought that upon dryingand calcining, the metal from the metal salt enters the pores of thesmall-pore molecular sieve, migrates to the Brønsted Acid sites viaconcentration gradient effect with or without the presence of H₂O vapor,then serving as a promoter metal, while the particles of zirconia (orother metal oxide) do not enter the pore structure of the molecularsieve. Instead, the zirconia forms an enrichment layer (zirconiacontaining layer) over the small-pore molecular sieve and/or betweenparticles, bounding them together to form an agglomerate of zeoliteparticles which are promoted with a metal.

In one or more embodiments, at least one binder compound is added to theaqueous based washcoat preparation following addition and dispersion ofthe molecular sieve powder, of which the particles have been enrichedwith zirconia. The washcoat, including the binder, is then applied to asubstrate, dried, and calcined to produce the final catalyst material.Such additional binder(s) can be selected from any binder known to thosein the art. In one or more embodiments, the additional binder can be atitania, alumina, zirconia, or silica binder known to those in the art.For example, without limitation, the binder can be selected fromtitanium oxychloride (TiOCl₂), titanium oxysulfate (TiOSO₄), aluminumtrihydrate (Al(OH)₃), boehmite (AlO(OH)), aluminum nitrate Al(NO₃)₃,SiO₂ sols (e.g. commercially available Nalco® 1034A), and zirconiacompounds.

Method of Reducing NO_(x) and Exhaust Gas Treatment System

In general, the molecular sieve material having a zirconia-containinglayer that is described above can be used as a molecular sieve,adsorbent, catalyst, catalyst support, or binder thereof. In one or moreembodiments, the material is used as a catalyst.

An additional aspect of the invention is directed to a method ofcatalyzing a chemical reaction wherein the catalyst of one or moreembodiments may be employed to catalyze a chemical reaction wherein thecatalyst is employed as catalytically active material.

Among others, said catalyst may be employed as catalyst for theselective reduction (SCR) of nitrogen oxides (NO_(x)); for the oxidationof NH₃, in particular for the oxidation of NH₃ slip in diesel systems.

One or more embodiments provide a method of selectively reducingnitrogen oxides (NO_(x)). In one or more embodiments, the methodcomprises contacting an exhaust gas stream containing NO_(x) with thecatalyst of one or more embodiments. In particular, the selectivereduction of nitrogen oxides wherein the selective catalytic reductioncatalyst comprises a washcoat including a small-pore molecular sievehaving a pore structure and a maximum ring size of eight tetrahedralatoms and containing a promoter metal, and a zirconia (or other metaloxide) containing layer on the small-pore molecular sieve containing thepromoter metal, of embodiments of the invention, is employed ascatalytically active material in a reaction carried out in the presenceof ammonia or urea.

While ammonia is the reducing agent of choice for stationary powerplants, urea is the reducing agent of choice for mobile SCR systems.Typically, the SCR system is integrated in the exhaust gas treatmentsystem of a vehicle and, also typically, contains the following maincomponents: selective catalytic reduction catalyst comprising a washcoatincluding a small-pore molecular sieve having a pore structure and amaximum ring size of eight tetrahedral atoms and containing a promotermetal, and a zirconia (or other metal oxide) containing layer on thesmall-pore molecular sieve containing the promoter metal according toembodiments of the invention and an injector to inject a reductant suchas ammonia or an ammonia precursor (e.g. urea) located upstream from theSCR catalytic article. In specific embodiments, the system can include aurea storage tank; a urea pump; a urea dosing system; a ureainjector/nozzle; and a respective control unit.

In other embodiments, the SCR catalyst according to one or moreembodiments is employed as an SCR catalyst in an exhaust gas treatmentsystem for lean-burn gasoline direct injection (GDI) engines. In suchcases, the SCR catalyst according to one or more embodiments serves as apassive ammonia-SCR catalyst and is able to store ammonia effectively attemperatures of 400° C. and above.

As used herein, the term “stream” broadly refers to any combination offlowing gas that may contain solid or liquid particulate matter. Theterm “gaseous stream” or “exhaust gas stream” means a stream of gaseousconstituents, such as the exhaust of a lean burn engine (i.e., an enginethat burns fuel in the presence of an excess of oxygen), which maycontain entrained non-gaseous components such as liquid droplets, solidparticulates, and the like. The exhaust gas stream of a lean burn enginetypically further comprises combustion products, products of incompletecombustion, oxides of nitrogen, combustible and/or carbonaceousparticulate matter (soot), and un-reacted oxygen and nitrogen.

The term nitrogen oxides, NO_(x), as used in the context of embodimentsof the invention designates the oxides of nitrogen, especiallydinitrogen oxide (N₂O), nitrogen monoxide (NO), dinitrogen trioxide(N₂O₃), nitrogen dioxide (NO₂), dinitrogen tetroxide (N₂O₄), dinitrogenpentoxide (N₂O₅), nitrogen peroxide (NO₃).

A further aspect of the invention is directed to an exhaust gastreatment system. In one or more embodiments, the exhaust gas treatmentsystem comprises an exhaust gas stream optionally containing a reductantlike ammonia, urea, and/or hydrocarbon, and in specific embodiments,ammonia and/or urea, and a selective catalytic reduction catalystcomprising a washcoat including a small-pore molecular sieve having apore structure and a maximum ring size of eight tetrahedral atoms andcontaining a promoter metal, and a zirconia (or other metal oxide)containing layer on the small-pore molecular sieve containing thepromoter metal according to one or more embodiments. The catalyst iseffective for destroying at least a portion of the ammonia in theexhaust gas stream.

In one or more embodiments, the catalyst can be disposed on a substrate,for example a soot filter. The soot filter, catalyzed or non-catalyzed,may be upstream or downstream of the catalyst. In one or moreembodiments, the system can further comprise a diesel oxidationcatalyst. In specific embodiments, the diesel oxidation catalyst islocated upstream of the catalyst. In other specific embodiments, thediesel oxidation catalyst and the catalyzed soot filter are upstreamfrom the catalyst.

In specific embodiments, the exhaust is conveyed from the engine to aposition downstream in the exhaust system, and in more specificembodiments, containing NO_(x), where a reductant, e.g. urea, is addedand the exhaust stream with the added reductant is conveyed to thecatalyst.

For example, a catalyzed soot filter, a diesel oxidation catalyst, and areductant are described in WO 2008/106519, which is herein incorporatedby reference. In specific embodiments, the soot filter comprises awall-flow filter substrate, where the channels are alternately blocked,allowing a gaseous stream entering the channels from one direction(inlet direction), to flow through the channel walls and exit from thechannels from the other direction (outlet direction).

An ammonia oxidation catalyst (AMOx) may be provided downstream of thecatalyst of one or more embodiments to remove any slipped ammonia fromthe system. In specific embodiments, the AMOx catalyst may comprise aplatinum group metal such as platinum, palladium, rhodium, orcombinations thereof.

Such AMOx catalysts are useful in exhaust gas treatment systemsincluding an SCR catalyst. As discussed in commonly assigned U.S. Pat.No. 5,516,497, the entire content of which is incorporated herein byreference, a gaseous stream containing oxygen, nitrogen oxides, andammonia can be sequentially passed through first and second catalysts,the first catalyst favoring reduction of nitrogen oxides and the secondcatalyst favoring the oxidation or other decomposition of excessammonia. As described in U.S. Pat. No. 5,516,497, the first catalystscan be a SCR catalyst comprising a zeolite and the second catalyst canbe an AMOx catalyst comprising a zeolite.

AMOx and/or SCR catalyst composition(s) can be coated on the flowthrough or wall-flow filter. If a wall flow substrate is utilized, theresulting system will be able to remove particulate matter along withgaseous pollutants. The wall-flow filter substrate can be made frommaterials commonly known in the art, such as cordierite, aluminumtitanate or silicon carbide. It will be understood that the loading ofthe catalytic composition on a wall flow substrate will depend onsubstrate properties such as porosity and wall thickness, and typicallywill be lower than loading on a flow through substrate.

One exemplary emissions treatment system is illustrated in FIG. 16,which depicts a schematic representation of an emission treatment system32. As shown, an exhaust gas stream containing gaseous pollutants andparticulate matter is conveyed via exhaust pipe 36 from an engine 34(e.g., a diesel engine, lean GDI engine, or other lean burn engine) to adiesel oxidation catalyst (DOC) 38 to a catalyzed soot filter (CSF) to aselective reductive catalyst (SRC), which is coated with the washcoatcomposition of the present invention. In the DOC 38, unburned gaseousand non-volatile hydrocarbons (i.e., the SOF) and carbon monoxide arelargely combusted to form carbon dioxide and water. In addition, aproportion of the NO of the NO component may be oxidized to NO₂ in theDOC.

The exhaust stream is next conveyed via exhaust pipe 40 to a catalyzedsoot filter (CSF) 42, which traps particulate matter present within theexhaust gas stream. The CSF 42 is optionally catalyzed for passive oractive soot regeneration. The CSF 42 can optionally include a SRCcomposition of the invention for the conversion of NO_(x) present in theexhaust gas.

After removal of particulate matter, via CSF 42, the exhaust gas streamis conveyed via exhaust pipe 44 to a downstream selective catalyticreduction component 46 of the invention for the further treatment and/orconversion of NO_(x). The exhaust gas passes through the SCR component46 at a flow rate which allows sufficient time for the catalystcomposition to reduce the level of NO_(x) in the exhaust gas at a giventemperature. The SCR component 46 may optionally be included in theemission treatment system when CSF 42 already includes an SCR catalystcomposition. An injector 50 for introducing a nitrogenous reducing agentinto the exhaust stream is located upstream of the SRC 46. Theintroduced nitrogenous reducing agent into the gas exhaust streampromotes the reduction of the NO_(x) to N₂ and water as the gas isexposed to the catalyst composition. If the CSF 42 also contains an SCRcatalyst, the injector 50 can be moved to a position upstream of theCSF.

The invention is now described with reference to the following examples.Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

EXAMPLES Example 1—10% ZrO₂ as Zirconyl Acetate Added to Slurry

Cu exchanged CHA (3.25 wt. % CuO, SAR 28) was dispersed in water,recycled through an in-line homogenizer @ 50 Hz to break largeagglomerates to D90<14 μm. Zirconyl acetate binder was then added toachieve total binder loading of 10 wt. % calcined washcoat basis. FinalpH of the resulting slurry was about 4.0. The mixture was then coatedonto a cordierite substrate, dried, and calcined to 450° C. to form theactive catalytic coating. Drying was accomplished using a forced fanheated oven to facilitate flow through the part. Because the zirconylacetate was highly soluble, it moved through the washcoat during dryingand formed an enrichment layer on the surface of the CHA. Final washcoatcomposition was 2.9% CuO, 87.1% CHA and 10% ZrO₂ upon coating, dryingand calcination.

Referring to FIG. 1, SCR conversion was relatively unchanged on the lowtemperature end, 200-300° C., and slightly higher as the hightemperature end for the 10% ZrO₂ version (relative to the same washcoatprepared with only 5% ZrO₂).

Zirconyl acetate solution also contained excess acetic acid forstability purposes (acetic acid/ZrO₂ mole ratio ˜1.6), so more CuObecame soluble and migrated to the surface, explaining the selectivitychange that was observed between 450-600° C. where performance of 10%ZrO₂ dropped, as free CuO (ex-exchange site) is known to oxidize NH₃ toactually form more NO_(x) vs. reducing what is already there.

As shown in FIG. 1, the enrichment of ZrO₂ on the washcoat surface isresponsible for reduced N₂O make at all temperatures (200-600° C.). Thisexperiment illustrates a correlation between zirconium oxideconcentration in the washcoat and N₂O make, with higher zirconium oxideamounts leading to reduced N₂O make.

Example 2—5% Zirconyl Acetate Plus Higher Slurry Conductivity

To explore impact of higher soluble Zr, one slurry batch was produced inthe same manner as Example 1 but with only 5 wt. % ZrO₂ added aszirconyl acetate. After the addition of zirconyl acetate, the slurry wassplit into two samples, designated as Sample 1 and Sample 2. Sample 1was not modified further, and, thus, had the same composition as one ofthe compositions of Example 1.

To Sample 2 was added 0.1% ammonium nitrate (NH₄NO₃) to increaseconductivity (as measured using a Cole Parmer Item# EW-19601-04).Conductivity increased from 870 μS/cm to 2200 μS/cm with final pH ofboth slurries being 4.4 (see Table 1).

TABLE 1 Solubility of Cu and Zr in aqueous liquid phase of the washcoatslurry post centrifugation Conductivity % Sample Solids pH (μS/cm)Element Soluble Sample 1 34.50 4.4 870 Cu 4.04 Low conductivity, Zr12.71 low Zr solubility Sample 2 37.00 4.4 2200 Cu 10.60 Highconductivity, Zr 27.24 high Zr solubility

Using same method as Example 1, the mixture was then coated onto acordierite substrate, dried, and calcined to 450° C. to form the activecatalytic coating. Drying was accomplished using a forced fan heatedoven to facilitate flow through the part. Table 1 shows that the Zrsolubility of Sample 2 was 2× higher than that of Sample 1. Solubilitywas measured by taking a sample of slurry, adding it to a centrifugetube and subjecting it to 7800 rpm for 2 hours using a Thermo ElectronCorporation IEC CL40R centrifuge. The resulting clear water white toblue colored supernatant (depending on how much Cu²⁺ was soluble) wasanalyzed using inductively coupled plasma (ICP) to determine parts permillion concentrations in the liquid phase, the % soluble fraction of agiven element in the liquid phase was calculated based on the slurrysolid content and elemental composition of the washcoat. Table 1 andTable 2 (see Example 3) demonstrate that the solubility of the Zr needsto be greater than 15% in the slurry phase to as high as 100% soluble inorder to facilitate migration of the soluble species (i.e., zirconiumcompound) to the surface of the washcoat during drying.

FIGS. 2 and 3 are scanning electron microscope (SEM) images of Sample 1and Sample 2, respectively, focused on the portion of the washcoat layerin a corner of a coated substrate cell. FIG. 2 (Sample 1) and FIG. 3(Sample 2) EDS tabular results both show higher Zr and Cu elementconcentration on the surface. FIG. 3, however, shows a visibleenrichment band containing higher Cu and Zr, which is tied directly tothe amounts of each element that are soluble in the aqueous slurryphase, as demonstrated in Table 1.

FIG. 4 is a bar chart comparing Sample 2 and Sample 1 in terms of NO_(x)efficiency when each sample was used in a specific emission treatmentsystem (with results reported as two measurements per sample and theaverage thereof). FIG. 4 shows that Sample 2 (higher Zr and Cu on thesurface, FIG. 3) has improved performance when compared to Sample 1(FIG. 2). The system results include use of same/standardized (aconstant) DOC as first catalyst in the system (6.5″ ϕ×8″ L with PGM at70 g/ft³), the second and third catalysts in the system were 8″ ϕ×6″ L400/4.5 coated with Cu/CHA (SCR) slurry, and the fourth catalyst in thesystem was the same/standardized (a constant) Catalyzed Soot Filter(CSF) measuring 8″ ϕ×10″ L. The complete system is for medium dutyapplication where low temperature performance is important. Prior totesting, the system was aged using a 6.7 L engine at 750° C. as measuredat the Diesel Oxidation Catalyst (DOC) outlet. Efficiencies weremeasured using EPA75 test cycle and reported as weighted modal data DOCto SCR outlet without any regeneration step. Temperature at which datais generated was between 180 and 220° C. with average ca. 200° C. wheredata is recorded.

This example confirms that the presence of soluble zirconium species inthe washcoat leads to increased migration of zirconium to the surface ofthe washcoat, which can improve low temperature NO_(x) reduction.However, as noted above, the increase in soluble zirconium species inthe outer portion of the washcoat is also accompanied by increasedcopper concentration in the same region, which is attributable toincreased copper migration during the washcoating process. Coppermigration in this manner can be detrimental to high temperature NO_(x)conversion.

Example 3—8% ZrO₂/3.25% CuO/CHA

Step 1: 1.7 Kg Cu (II) nitrate crystal was dissolved in 3.6 kgcommercially available nitric acid based zirconia sol with ZrO₂ contentof 15% by weight by mixing at room temperature.

Step 2: The solution from step 1 was impregnated onto 18.8 kg spraydried NH₄/CHA powder then simultaneously dried/calcined. The product ofthis step, a 3% ZrO₂/3.25% CuO/CHA powder, is shown in FIG. 5 as ascanning electron microscope (SEM) image with magnification at 10000×.The SEM image shows the presence of the zirconium particles (lightercolored material) surrounding, and dispersed among, the zeoliteparticles (darker, larger particles).

Step 3: Calcined powder from step 2 was then dispersed in water,recycled through an in-line homogenizer @ 50 Hz to break largeagglomerates to D90<14 μm. An additional 5% by wt. zirconyl acetate asbinder was added to achieve total ZrO₂ loading of about 8% wt. %calcined, on a washcoat basis. The final pH of the resulting slurry was3.8.

The mixture was then coated onto a cordierite substrate, dried, andcalcined to 450° C. to form the active catalytic coating. Drying wasaccomplished using a forced fan heated oven to facilitate flow throughthe part. The final washcoat composition was 3.1% CuO, 89.1% CHA, and7.8% ZrO₂ upon coating, drying, and calcination, and is designated asSample 3 in Table 2. Table 2 shows that Sample 3 contained low Cu and Zrsolubility due to a pre-preparation step which included fastdrying/calcination of the 3% ZrO₂/3.25% CuO/CHA powder preparation step.This fast drying and calcination in air created a rapid concentrationgradient in <1.5 seconds, which created a driving force to move the Cu²⁺to the Brønsted acid sites.

Sample 4 was prepared by dispersing Cu exchanged CHA (3.25 wt. % CuO,SAR 28) in water, recycled through an in-line homogenizer @ 50 Hz tobreak large agglomerates to D₉₀<14 μm. Zirconyl acetate binder was thenadded to achieve total binder loading of 5 wt. % on a calcined washcoatbasis. Final pH of the resulting slurry was about 4.3. The mixture wasthen coated onto a cordierite substrate, dried, and calcined to 450° C.to form the active catalytic coating. Final composition of Sample 4 was3.1% CuO/5.0% ZrO₂/91.9% CHA. Sample 4 contained higher soluble Cu andZr in the slurry phase when compared to Sample 3.

TABLE 2 Solubility of Cu and Zr in aqueous liquid phase of the washcoatslurry post centrifugation Sample Solids pH Element % Soluble Sample 343.00 3.8 Cu 1.40 Zr 5.62 Sample 4 48.00 4.3 Cu 3.57 Zr 26.72

FIGS. 6A-6D are a collection of scanning electron microscope (SEM)images of the material of Sample 4. The top left box (6A) is an SEMphotomicrograph showing washcoat distribution within the unit substratecell with magnification 50×. Going counterclockwise, the bottom left box(6B) shows distribution of Cu in the washcoat using Electron DispersiveSpectroscopy (EDS) mapping, and illustrates that Cu is distributedevenly through the washcoat layer with slight enrichment at the surfaceof the coating. Continuing counterclockwise, the bottom right cornerphotomicrograph (6C) shows Zr distribution via EDS. There is adistinctive enrichment band of Zr at the surface of the washcoat, whichcorrelates with Zr solubility in Table 2. At the top right corner isanother SEM photomicrograph (6D) at 10,000×, focusing in on the Zrenrichment layer on the surface which is about 1-3 μm thick. FIGS. 6A-5Ddemonstrate enrichment of ZrO₂ on the surface of the washcoat as alsoobserved for Examples 1 and 2.

FIGS. 7A-7D are a collection of scanning electron microscope (SEM)images of the material of Sample 3. These figures demonstrate that thesample with reduced amounts of soluble Cu and Zr did not form anyenrichment layer; however, some layering and particle to particlebonding with nano-ZrO₂ is noted, and the washcoat also appears to bemore porous. The top left corner (7A) shows washcoat distribution withinthe unit substrate cell with magnification 50×. Continuingcounterclockwise, the bottom left corner (7B) demonstrates that Cu isuniformly dispersed throughout the washcoat. The bottom right corner(7C) shows that Zr is dispersed throughout the washcoat with areas ofhigher concentration (brighter areas), such areas are also present inother samples but they are more prevalent as Zr concentration increases.No enrichment of Zr is noted on the surface of the washcoat layer.Ending with the upper right corner (7D), Zr is clearly evident on thesurface of some particles and forming bridges between particles. Thecoating also appears to be more porous. It is thought that fineparticles of CHA are bound together with larger ones during the flashdried/calcined step during preparation of the ZrO₂/Cu/CHA compositepowder step. Zirconyl acetate then added during the slurry prep/washcoatfabrication step further binds particles together during the coatedsubstrate drying and calcination steps to form the final 7.8% ZrO₂/3.1%CuO, 89.1% CHA washcoat layer.

Table 3 outlines the reactor gas composition and test protocol.

TABLE 3 SCR CAEF # of Reactor Passes 1 Core Width (mm) 24.9 Core Length(mm) 76.2 Test Temperatures (° C.) Temp. 1 250 Temp. 2 525 Run GasesConcentration/Flow Concentration Flow N₂ %/(l/min) N₂ 9.32 O₂ %/(l/min)10.00 9.37 Ammonia ppm/(l/min) 500 0.52 NO_(x) ppm/(l/min) 500 0.52Water %/(l/min) 5% 1.0 Total gas flow (l/min) 20.76 Space Velocity(hrs⁻¹) 50006

FIG. 8 is a bar chart showing the NO_(x) reduction at 200, 250, and 525°C. for the material of Sample 3 compared to Sample 4, which contains Zrenrichment on the surface of the washcoat. The samples were first agedat 700° C. in 10% steam and air for 4 hours. They were then tested in areactor, per Table 3. FIG. 8 demonstrates that reduction of NO_(x) at200° C. and 250° C. were marginally improved, with NO_(x) conversionsignificantly higher at 525° C. The broader performance window exhibitedby Sample 3 (higher zirconium loading with less soluble zirconiumspecies) is indicative of an improved exchange of Cu²⁺ to the Brønstedacid sites and the benefit of the thermal fixation step prior toincorporating the 3% ZrO₂/3.25% CuO/CHA powder to produce the finalwashcoat with composition 7.8% ZrO₂/3.1% CuO/89.1% CHA on N400/4substrate.

FIG. 9 is a bar chart showing the NH₃ Slip, NH₃ Storage, and N₂O make atvarying temperatures for the materials of Example 3. FIG. 9 demonstratesthat N₂O has been further reduced for the material of Sample 3 whencompared to the material of Sample 4. This can be explained as Sample 3contains a higher ZrO₂ concentration (8% versus 5% for Sample 4) withenrichment on the particle level, reduced soluble Cu and Zr in theslurry phase coupled with more effective exchange of Cu²⁺ to theBrønsted acid sites. This example shows that the benefits of increasingzirconium oxide concentration in the washcoat (such as reduced N₂O makeand improved NO_(x) reduction at low temperature) can be achievedwithout causing undesirable copper migration by reducing reliance onsoluble zirconium species to obtain the increased zirconium oxideconcentration.

Example 4: 5% Nano-Ceria/Zirconia on 3.25% CuO/CHA

3.8 Kg of 3.25% CuO/CHA is first dispersed in 6.2 Kg of water thenrecycled through an in-line homogenizer @ 50 Hz to break largeagglomerates to a achieve a particle size distribution with D₉₀<20 μm.To this mixture 794 grams of an aqueous dispersion of (Ce₄₅Nd₅Zr₅₀)O₂ asdefined in Table 4 is added to the mixture and recirculation through thehomogenizer is continued until the particle size distribution has D90<14μm. The pH of the final slurry was 4.6.

The mixture was then coated onto a cordierite substrate, dried, andcalcined to 450° C. to form the active catalytic coating. Drying wasaccomplished using a forced fan heated oven to facilitate flow throughthe part. The final washcoat composition was 2.25% CeO2, 0.25% Nd2O3,2.51% ZrO2, 3.09% CuO and 91.91% CHA upon coating, drying, andcalcination, and is designated as Sample 5 in Table 5.

Sample 6 was prepared by dispersing Cu exchanged CHA (3.25 wt. % CuO,SAR 28) in water, recycled through an in-line homogenizer @ 50 Hz tobreak large agglomerates to D₉₀<14 μm. Zirconyl acetate binder was thenadded to achieve total binder loading of 5 wt. % on a calcined washcoatbasis. Final pH of the resulting slurry was about 4.3. The mixture wasthen coated onto a cordierite substrate, dried, and calcined to 450° C.to form the active catalytic coating. Final composition of Sample 6 was3.1% CuO/5.0% ZrO₂/91.9% CHA.

TABLE 4 Aqueous dispersion of <1 μm particles of ZrO₂ doped with CeO₂and Nd₂O₃ Test Nano-CeO₂/Nd₂O₃/ZrO₂ Units ZrO₂ + HfO₂ (bal.) 50.1 %,oxide basis CeO₂ 44.9 %, oxide basis Nd₂O₃ 5.0 %, oxide basis SolidsContent 25.2 % of dispersion pH 4.96

TABLE 5 Solubility of Cu and Zr in the aqueous liquid phase of thewashcoat slurry post centrifugation Sample Solids pH Element % SolubleSample 5 34.0 4.6 Cu not detected Zr not detected Sample 6 33.0 4.3 Cu3.37 Zr 14.77

FIGS. 10A-10D are a collection of scanning electron microscope (SEM)images of the material of Sample 5. The top left box (10A) is an SEMphotomicrograph showing washcoat distribution within multiple unitsubstrate cells with magnification 25×. Going counterclockwise, thebottom left box (10B) shows that distribution of Ce in the washcoatusing Electron Dispersive Spectroscopy (EDS) mapping, and it shows Cedistributed evenly through the washcoat. It is inferred that both Zr andNd are also well dispersed based on the composition of the sol asdefined in Table 4. Continuing counterclockwise, the photomicrograph onthe bottom right (10C) shows Cu is uniformly dispersed through thewashcoat and finally the top right photomicrograph (10D) shows porosityis evident in the coating once magnified to 500×.

Referring to FIG. 11, SCR conversion was marginally improved at 200-250°C. with Sample 5 but was marginally inferior to Sample 6 at temperaturesof between 250-600° C. FIG. 12 shows that Sample 5 is slightly inferiorto Sample 6 in terms of N₂O make. Although not bound by a theory ofoperation, it is suspected that the oxidative properties of CeO₂ are atleast partially responsible for this result. However, it is thought thatthe use of a ceria-zirconia composite material will have improvedqualities in terms of oxidizing soot during regenerations, therebyminimizing fouling of the zeolite. Additionally, metal oxide compositescombining greater amounts of zirconia as compared to ceria could achievethe desired reduction in N₂O make.

Example 5: 6% ZrO₂ on 3.25% CuO/CHA

1.7 Kg Cu (II) nitrate crystal was dissolved in 7.2 kg commerciallyavailable nitric acid based zirconia sol with ZrO₂ content of 15% byweight (ZSL-15N available from Daiichi Kigenso Kagaku Kogyo Co., Ltd) bymixing at room temperature. The resulting solution was impregnated onto18.2 kg spray dried NH₄/CHA powder in a mixer, then simultaneouslydried/calcined. The product of this step is a 6% ZrO₂/3.25% CuO/CHApowder. The calcined powder was then dispersed in water, recycledthrough an in-line homogenizer @ 50 Hz to break large agglomerates toD90<14 μm. This fast drying and calcination in air created a rapidconcentration gradient in <1.5 seconds, which, without being bound bytheory, is believed to create a driving force to move the Cu²⁺ to theBrønsted acid sites.

FIG. 13 shows powder reactor temperature sweep comparing standard 3.25%ion exchanged Cu/CHA material to 6% ZrO₂/3.25% CuO/CHA prepared in thisExample, with the lefthand y-axis providing % conversion of NO_(x) andthe righthand y-axis providing N₂O make in ppm. In the graph, thetriangles denote the NO_(x) conversion of the zirconia-modifiedmaterial, the square denotes the NO_(x) conversion of the comparativeCu/CHA material, the circle denotes the N₂O make for thezirconia-modified material, and the diamond denotes the N₂O make for thecomparative CuCHA material. As shown, the modification of the CHAmaterial with zirconia provides improved NO_(x) conversion at lowtemperature and slightly elevated N₂O make at low temperature. Hightemperature NO_(x) performance is roughly the same for both materials,with the zirconia-modified material providing improved N₂O make athigher temperature.

Example 6: 6% Al₂O₃ on 3.25% CuO/CHA

1.7 Kg Cu (II) nitrate crystal was dissolved in 5.4 kg commerciallyavailable nitric acid based alumina sol with Al₂O₃ content of 20% byweight (Dispal 23N4-20 available from Sasol), a large crystal boehmitematerial, by mixing at room temperature. The resulting solution wasimpregnated onto 18.2 kg spray dried NH₄/CHA powder by spraying thesolution onto the CHA powder in a mixer, then simultaneouslydried/calcined. The product of this step is a 6% Al₂/O₃/3.25% CuO/CHApowder. The calcined powder was then dispersed in water, recycledthrough an in-line homogenizer @ 50 Hz to break large agglomerates toD90<14 μm. This fast drying and calcination in air created a rapidconcentration gradient in <1.5 seconds, which, without being bound bytheory, is believed to create a driving force to move the Cu²⁺ to theBrønsted acid sites.

The catalyst materials of Example 5 and Example 6 were hydrothermallyaged at 800° C. for 6 hours in the presence of 10% H₂O and tested forNO_(x) conversion performance in comparison to an unmodified CuO/CHAmaterial aged under the same conditions. The results are set forth inFIG. 14, with the CuO/CHA material that does not contain a zirconia oralumina sol represented with a diamond. The catalyst material of Example5 (6% ZrO₂/3.25% CuO/CHA) is represented by a triangle and the catalystmaterial of Example 6 (6% Al₂/O₃/3.25% CuO/CHA) is represented by asquare. As shown in the figure, the CHA materials modified with zirconiaor alumina outperform the unmodified material at higher temperature.

Example 7: 8% ZrO₂/2% Y₂O₃ on 4.4% CuO/CHA

0.5 Kg of commercially available 60/40 ZrO2/Y2O3 mixed sol is dispersedin 41 kg of DI water which contains 0.17 kg 90% acetic acid. 2.2 Kg ofcommercially available nitric acid based zirconia sol with ZrO₂ contentof 15% by weight (ZSL-15N available from Daiichi Kigenso Kagaku KogyoCo., Ltd) is added to the dispersion created in previous step by mixingat room temperature. 5.4 Kg of spray dried 4.91% CuO/CHA is added to theresulting dispersion, producing 8% ZrO₂/2% Y₂O₃/4.40% CuO/CHA. Theproduct mixture from the previous step is then recycled through anin-line homogenizer @ 50 Hz to break large agglomerates to D90<14 μm.The resulting slurry is then coated onto 400/4 cordierite substrate,dried and calcined at 450° C. to achieve a dry gain of 2.75 g/in³.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the materials and methods discussed herein(especially in the context of the following claims) are to be construedto cover both the singular and the plural, unless otherwise indicatedherein or clearly contradicted by context. Recitation of ranges ofvalues herein are merely intended to serve as a shorthand method ofreferring individually to each separate value falling within the range,unless otherwise indicated herein, and each separate value isincorporated into the specification as if it were individually recitedherein. All methods described herein can be performed in any suitableorder unless otherwise indicated herein or otherwise clearlycontradicted by context. The use of any and all examples, or exemplarylanguage (e.g., “such as”) provided herein, is intended merely to betterilluminate the materials and methods and does not pose a limitation onthe scope unless otherwise claimed. No language in the specificationshould be construed as indicating any non-claimed element as essentialto the practice of the disclosed materials and methods.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It will be apparent to those skilled in the art thatvarious modifications and variations can be made to the method andapparatus of the present invention without departing from the spirit andscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A catalyst composition suitable for use as aselective catalytic reduction catalyst, comprising: small-pore molecularsieve particles having a pore structure and a maximum ring size of eighttetrahedral atoms and impregnated with a promoter metal, and metal oxideparticles dispersed within the small-pore molecular sieve particles andexternal to the pore structure of the small-pore molecular sieveparticles, wherein the metal oxide particles comprise one or more oxidesof a transition metal or lanthanide of Group 3 or Group 4 of thePeriodic Table.
 2. The catalyst composition of claim 1, wherein themetal oxide particles comprise a metal oxide selected from the groupconsisting of zirconia, alumina, ceria, hafnia, yttria, and combinationsthereof.
 3. The catalyst composition of claim 1, wherein the metal oxideparticles comprise zirconia.
 4. The catalyst composition of claim 1,wherein the metal oxide particles have an average particle size in therange of about 10 nm to about 500 nm.
 5. The catalyst composition ofclaim 1, wherein the metal oxide particles have a D₁₀ particle sizegreater than ten times larger than a pore opening of the molecularsieve.
 6. The catalyst composition of claim 1, wherein the metal oxideparticles have a D₁₀ particle size of about 10 nm or greater.
 7. Thecatalyst composition of claim 1, wherein the small-pore molecular sievehas a d6r unit.
 8. The catalyst composition of claim 1, wherein thesmall-pore molecular sieve has a structure type selected from AEI, AFT,AFX, CHA, EAB, ERI, KFI, LEV, LTN, MSO, SAS, SAT, SAV, SFW, and TSC. 9.The catalyst composition of claim 1, wherein the promoter metal isselected from the group consisting of Cu, Co, Ni, La, Mn, Fe, V, Ag, Ce,Nd, Pr, Ti, Cr, Zn, Zn, Nb, Mo, Hf, Y, W, and combinations thereof. 10.The catalyst composition of claim 1, wherein the small-pore molecularsieve has the CHA structure type.
 11. The catalyst composition of claim1, wherein the promoter metal comprises Cu or Fe or combinationsthereof.
 12. The catalyst composition of claim 1, wherein the promotermetal is present in an amount in the range of about 1 to about 10% byweight, based on the total weight of the molecular sieve.
 13. Thecatalyst composition of claim 1, wherein the promoter metal is presentin an amount in the range of about 2 to about 5% by weight, based on thetotal weight of the molecular sieve.
 14. The catalyst composition ofclaim 1, wherein the metal oxide is present in an amount in the range ofabout 1 to about 15% by weight, on an oxide basis, based on the totalweight of the washcoat.
 15. A catalyst article comprising a substrateselected from a flow-through monolith, a wall-flow filter, a foam, or amesh, wherein a catalyst composition of claim 1 is adhered as a washcoatlayer on the substrate.
 16. The catalyst article of claim 15, whereinthe washcoat is disposed on a flow-through monolith or a wall-flowfilter.
 17. The catalyst article of claim 15, wherein the catalystarticle is characterized by an N₂O make that is at least 10% by weightlower as compared to a catalyst article comprising a washcoat with thesame catalyst composition at the same loading but without metal oxideparticles dispersed within the small-pore molecular sieve particles. 18.A method for selectively reducing nitrogen oxides (NO_(x)), the methodcomprising contacting an exhaust gas stream containing NO_(x) with thecatalyst article of claim
 15. 19. The method of claim 18, wherein theamount of N₂O produced as a byproduct is reduced as compared to theamount of N₂O produced in a method using a catalyst article comprising awashcoat with the same catalyst composition at the same loading butwithout metal oxide particles dispersed within the small-pore molecularsieve particles.
 20. An exhaust gas treatment system comprising thecatalyst article of claim 15 downstream from an engine and an injectorthat adds a reductant to the exhaust gas stream.
 21. A method ofpreparing a catalyst composition, the method comprising: dissolving asalt of at least one promoter metal in an aqueous-based metal oxide sol,wherein the salt of the at least one promoter metal dissociates in theaqueous-based metal oxide sol to form an aqueous-based metal salt/metaloxide sol mixture, wherein the metal oxide particles comprise one ormore oxides of a transition metal or lanthanide of Group 3 or Group 4 ofthe Periodic Table; treating ammonium or proton exchanged small-poremolecular sieve particles having a pore structure and a maximum ringsize of eight tetrahedral atoms with the aqueous-based metal salt/metaloxide sol mixture to allow impregnation of the promoter metal into thepore structure of the small-pore molecular sieve; and drying andcalcining the treated small-pore molecular sieve particles to form thecatalyst composition, wherein the catalyst composition comprises thesmall-pore molecular sieve particles impregnated with the promotermetal, and metal oxide particles dispersed within the small-poremolecular sieve particles and external to the pore structure of thesmall-pore molecular sieve particles.
 22. The method of claim 21,wherein the metal oxide is selected from the group consisting ofzirconia, alumina, ceria, hafnia, yttria, and combinations thereof. 23.The method of claim 21, wherein the metal oxide comprises zirconia. 24.The method of claim 21, wherein the metal oxide sol has an averageparticle size in the range of about 10 nm to about 500 nm.
 25. Themethod of claim 21, wherein the metal oxide sol has a D₁₀ particle sizegreater than ten times larger than a pore opening of the molecularsieve.
 26. The method of claim 21, wherein the metal oxide sol has a D₁₀particle size of about 10 nm or greater.
 27. The method of claim 21,wherein the promoter metal is selected from the group consisting of Cu,Co, Ni, La, Mn, Fe, V, Ag, Ce, Nd, Pr, Ti, Cr, Zn, Zn, Nb, Mo, Hf, Y, W,and combinations thereof.
 28. The method of claim 21, wherein the metaloxide sol is selected from the group consisting of zirconyl hydroxidesols, nano-sized hydrous zirconia sols, alumina sols, zirconia-yttriasols, zirconia-alumina sols, zirconia-ceria sols, organo-zirconium sols,and mixtures thereof.
 29. The method of claim 21, wherein metal oxideparticles do not enter the pore structure of the small-pore molecularsieve.
 30. The method of claim 21, wherein the small-pore molecularsieve has a d6r unit.
 31. The method of claim 21, wherein the small-poremolecular sieve has a structure type selected from AEI, AFT, AFX, CHA,EAB, ERI, KFI, LEV, LTN, MSO, SAS, SAT, SAV, SFW, and TSC.
 32. Themethod of claim 21, wherein the small-pore molecular sieve has the CHAcrystal structure.
 33. The method of claim 21, wherein the promotermetal comprises Cu, Fe, or a combination thereof.
 34. The method ofclaim 21, further comprising the steps of mixing the catalystcomposition with water to form a washcoat slurry; applying the washcoatslurry to a substrate to form a washcoat coating thereon; and drying andcalcining the substrate to form a catalytic article.
 35. The method ofclaim 34, further comprising adding a water soluble metal oxide compoundto the washcoat slurry to increase the total metal oxide contentthereof.